1. Field of the Invention
The present invention relates to aluminum heat exchangers and parts thereof having improved properties, and more specifically to such heat exchangers and parts which exhibit increased strength at elevated temperatures.
2. Description of Related Art
Several aluminum alloys are currently used in construction of heat exchangers. The 20 most common are alloys typified by an alloy bearing designation AA 3003. This alloy has low strength, and is easily formed to sheet, fins and tubes. Slightly higher strengths are obtained by heat exchanger parts constructed using alloy AA 3005. The AA 3003 alloy has good corrosion resistance and was initially employed in radiators. Thereafter it was used in construction of heat exchangers such as charge air coolers (inter-coolers) on automobiles equipped with turbochargers.
Turbochargers use the engine exhaust gas to turn a turbine which drives a compressor forcing air into engine's piston cylinder chamber, increasing combustion efficiency which improves fuel efficiency and performance. The compression of the intake air increases its temperature, somewhat reducing the beneficial effects of the air compression. For this reason, it is beneficial to cool the intake air compressed by the turbocharger prior to its injection into the cylinder chamber. This is done by employing an air to air heat exchanger (known in the automobile and truck industry as an inter-cooler or charge-air-cooler). The now cooled and compressed air results in maximum performance derived from turbocharging, lowering emission levels and improving fuel efficiencies. In North America turbochargers are generally used only on specialty automobiles, but they are employed on almost all heavy trucks and construction vehicles. Because the benefits of turbocharging increase as the pressure of the intake air increases, there is a desire to further increase the output pressure of turbochargers. Such a pressure increase is accompanied by a proportional increase in gas temperature, in accordance with Boyle's gas law. This, in turn, places increased temperature demand on the charge-air-coolers.
The new 2nd & 3rd generation alloys that have been developed for use in heat exchangers had as objectives, improved corrosion resistance, improved brazeability and increased strength. Excellent corrosion resistance was obtained by increasing the % Cu to the existing alloys. Heat exchangers constructed using the modified alloy 3190 exhibited improved strength; a similar alloy is MD 356, which also has Ti additions to further increase its corrosion resistance.
In heat exchanger and radiator construction, the alloys AA 3003, AA 3005, MD 356 and 3190 are the core alloys currently used; the compositions of these alloys are given in Table 1. These structural core alloys are joined into a component by brazing. The braze alloys are clad on one or both sides of the core alloy. The braze alloys typically have an Al--Si eutectic base, which falls between the 7% Si or 12% Si composition range. For radiator applications, the water side is clad with "pure" aluminum that further improves the corrosion resistance. Cladding thickness varies from 5 to 15% of the core alloy thickness. Because significant diffusion occurs when the braze clad Al--Si melts it is necessary that the braze clad be compatible with the core alloy. The solidus of the core alloy must also be above the braze temperature. These requirements limit the type of alloy that may be used as the core.
TABLE I __________________________________________________________________________ Mn Mg Cu Si Fe Cr Zn Ti Alloy wt %! wt %! wt %! wt %! wt %! wt %! wt %! wt %! __________________________________________________________________________ AA3003 1.0-1.5 0.1 max 0.05-0.20 0.6 max 0.7 max -- 0.1 max -- AA3005 1.0-1.5 0.20-0.60 0.3 max 0.6 max 0.7 max 0.1 max 0.25 max 0.1 max 3190 1.0-1.5 0.3-0.7 0.3 max 0.4 max 0.4 max -- -- -- MD356 0.8-1.3 0.4-0.6 0.30-0.55 0.25 max 0.4 max -- 0.1 max 0.11-0.20 __________________________________________________________________________
The 2nd & 3rd generation aluminum alloys, such as the proprietary MD356 and 3190 alloys are limited to temperatures of 177.degree. C. 350.degree. F!, as are the AA3003 & AA3005 alloys on which they are based. This temperature limitation seriously restricts the potential benefits of turbocharging systems. Metals other than aluminum could be used for charge air coolers to improve the elevated temperature strength, just as copper is used for radiators. However, the increase in weight encountered in use of those metals would offset any benefit derived from the increased power afforded by their ability to operate at higher temperatures.
A charge air cooler or intercooler is conventionally comprised of a side plate, header, tubes and fins. Hot gas is fed to the tubes by the header which is basically a manifold that is held in place by a side plate. The fins are thin sheets attached to the tubes in order to promote cooling. The cool air flows over the fins and the outside of the tubes, while the hot gas flows inside the tubes and the manifold. The tubes and manifold are therefore hotter than the fins. All of these parts are brazed together, with the result that any alloy used must be able to withstand an approximate 600.degree. C. braze cycle.
AlliedSignal Turbocharging Systems Division (ASTS) charge-air-coolers are constructed using the 3190 alloy for the header and tubes and AA3 003 for the less demanding fins. These give satisfactory service under current operating conditions. However, it has been found by ASTS that the 3190 and similar alloys tend to fail by thermal fatigue when used in charge-air-coolers designed for temperatures in excess of 177.degree. C.
ASTS have calculated the stress and temperature distribution in a charge air cooler on a Freightliner DDA-960-470 HP engine subject to the most severe conditions it would encounter during passage over the Rocky Mountains using the Loveland Pass. It was found that the first tube connection to the header would experience the maximum temperature and stress. The stress at this location is higher, because it is the location at which the header is connected to the cooler side plate, at this location the header is not only cooler, but is also firmly fixed in position. The stress experienced occurs not only during the acceleration cycle, but also during deceleration, when the temperature differential is reversed. ASTS found that the temperature reaches 405.degree. F. and the effective stress is 4.0 ksi. Comparison with fatigue curves shows that theoretically the header tube connection will fail by a low cycle failure after an unacceptable 6,000 "Loveland Pass" cycles. Such failures were found by ASTS in the new aircooler intended for operation at over 177.degree. C. Another failure mode is bending of the side plate. Analysis indicates that although the temperature of the side plate is only around 150.degree. C., the stress is high, with calculated values of 9 ksi, hence its failure mode is typical of an overloaded structure rather than a fatigue failure.
Heat exchangers constructed using the stronger AA 6000 type alloys have been proposed; but these devices have been found to over-age at temperatures above 177.degree. C., thereby causing rapid (&lt;100 hours), deleterious loss of mechanical properties with time.
There remains a need in the art for a heat exchanger, the parts of which exhibit useable strength after long term exposure to temperatures above 177.degree. C. and after experiencing a standard braze cycle.